The present invention relates in general to communication systems and components, and is particularly directed to a periodic waveform-based wireline measurement circuit architecture, that is readily suited for incorporation into a subscriber line interface circuit (SLIC). This architecture is configured to derive a voltage that is very precisely proportional to the differential voltage across the tip and ring leads, and thereby representative of the resistance, of a wireline pair, and to modulate the width of a periodic waveform, such as one derived from a readily available ringing signal, in accordance with the derived voltage.
As described in the above-referenced ""324 application, a variety of equipments employed by telecommunication service providers employ what are known as subscriber line interface circuits or xe2x80x98SLIC""s, which interface (transmit and receive) communication signals with tip and ring leads of a wireline pair, to which a (remote) piece of subscriber equipment is connected. This wireline pair is not only used to transport AC signals (e.g., voice and/or ringing), as well as substantial DC voltages, but its length can be expected to vary from installation to installation, and may be relatively long (e.g., on the order of multiple miles).
For optimized signal transmission and reception, the SLIC is designed to compensate for signal distortion characteristics of the line (including attenuation). Because distortion characteristics strongly depend upon the length of the line, measuring the differential voltage across its tip and ring leads is usually performed to obtain an indication of line length. However, as the differential line voltage may be quite large, conventional loop voltage measurement circuits cannot be readily incorporated into present day SLIC architectures, that employ transmission channels which must conform with very demanding performance requirements.
These requirements include accuracy, linearity, signals, low noise, filtering, low power consumption, insensitivity to common mode, and ease of impedance matching programmability, that allow the SLIC to be interfaced with a variety of telecommunication circuits, including those providing digital coder/decoder (codec) functionality. Digital signal processor (DSP)-based codecs are currently being used with increasing frequency in telecommunication circuits, due to their ability to automatically compensate for many of the limitations presented by the phone line. Thus, providing line length information in a format that is easily interpreted by the codec function is very desirable feature of any SLIC.
In accordance with the present invention, this objective is successfully addressed by means of a new and improved wireline measurement circuit architecture, that is designed to be incorporated into a subscriber line interface circuit, and is operative to derive a voltage that is very precisely proportional to the differential voltage across the line""s tip and ring leads, and thereby representative of the resistance, of a wireline pair. This derived voltage is then used to modulate the width of a periodic waveform, such as one derived from the ringing signal, in accordance with the derived precision differential tip-ring voltage. With the measured line characteristic information being infused into a periodic waveform, it can be readily interfaced in an asynchronous manner with digital processing components, such as, but not limited to, a DSP codec. The use of a periodic waveform also allows for averaging, so that instantaneous deviations caused by signals on the line can be removed.
As will be described, the duty cycle of successive pulse signals of the periodic waveform (derived from the ringer signal) is modulated in accordance with the differential tip-ring voltage measured between the tip and ring leads of the telephone line. The pulses of the periodic waveform derived from the ringer signal vary in amplitude between first (e.g. relatively positive) and second (e.g., relatively negative) states, with the widths of one of the states (e.g., the relatively negative state) proportional to the instantaneous voltage across the tip and ring terminals to which the wireline being measured is connected. Since differential tip-ring voltage increases with line length, the higher the measured differential voltage, the longer the line (and higher associated resistance), and therefore the wider are the relatively negative states of the pulses, making the duty cycle of the pulse train signal directly proportional to line length. As the current limit setting for the SLIC is known, it is easy to calculate the line resistance of a loop operating at the current limit, as well as to establish a lower bound for the line length for a loop which operates below the current limit.
In order to measure the differential tip-ring voltage, the present invention preferably incorporates a precision loop voltage detector of the type disclosed in the above-referenced ""324 application. As described therein, and as will detailed below, this precision loop detector couples a front-end, a complementary transistor pair-configured tip-ring sensing rectifier circuit to the tip and ring leads of the wireline being measured. These complementary transistor pairs are intercoupled through a relatively high valued tip-ring voltage sensing resistor and have a collector-emitter current output path that supplies an output current containing a composite of two voltage-representative current components.
The first voltage-representative current component is representative of the desired differential tip-ring voltage. The second voltage-representative current component (which constitutes an undesired offset) is associated with the internal characteristics (in particular, the base-emitter voltage drops of complementary transistors) of the tip-ring voltage detector. The differential current extraction circuit serves to separate the second current component from the composite current.
Each of the second current component and the composite current is then appropriately scaled, so that when differentially recombined, the scaled version of the second voltage-representative current component (associated with the base-emitter voltages of complementary transistors of the tip-ring voltage detector) is canceled from the composite current, leaving only a current component representative of the differential tip-ring voltage.
The differential current extraction circuit contains a pair of current mirror circuits, and an auxiliary voltage/current reference circuit. A first of the current mirrors generates a first mirrored current component that is fractionally proportional to the composite output current produced by the tip-ring sensing rectifier circuit, and thereby representative of the sum of the differential tip-ring voltage and the base-emitter voltage drops of the complementary pair of transistors of the tip-ring sense rectifier circuit.
The first current mirror also outputs a mirrored current to the auxiliary voltage/current reference circuit. This auxiliary voltage/current reference circuit is comprised of a pair of series-connected, complementary transistors that are coupled across a relatively large valued scaling resistor. The geometries of the transistors of the auxiliary voltage/current reference circuit are such that they operate at the same current densities as the transistor pairs of the tip-ring sensing rectifier.
As a result, the output voltage produced by the auxiliary voltage reference is representative of only the base-emitter voltage drops of one of the complementary pairs of transistors of the rectifier. Therefore, a resultant auxiliary current flowing through the scaling resistor, across which the voltage produced by the auxiliary voltage reference circuit is coupled, corresponds to only the base-emitter voltage drops of a complementary pair of transistors of the tip-sense rectifier circuit.
The auxiliary current is then mirrored and scaled by a second of the two current mirrors as a second current component, that is fractionally proportional to only the base-emitter voltage drops of the complementary pair of transistors of the tip-ring sense rectifier circuit. The two mirrored and scaled currents are then combined at a single ended tip-ring voltage measurement node to which a voltage-dropping output-scaling resistor that is connected. This results in the base-emitter voltage drop components canceling one another, so as to produce a net output current through and thereby tip-ring output voltage across the voltage-dropping output-scaling resistor that is precisely representative of only the differential tip-ring voltage, as desired.
In order to modulate the duty cycle of the waveform derived from the ringing signal in proportion to line length, the precision tip-ring output voltage produced at the single ended tip-ring voltage measurement is coupled to the inverting input of a voltage comparator of a periodic waveform modulator. The comparator has its non-inverting input coupled to a ground-referenced integration capacitor, to which a current source is coupled. The non-inverting input of the comparator is further coupled to a first controlled switch that is selectively closed to connect the non-inverting input of the comparator to ground, or remains open, in accordance with the polarity of the ringing signal. When the ringer voltage is at or above ground potential, the first controlled switch is closed, so as to couple the non-inverting input of the comparator to a zero or logic low potential. On the other hand, when the ringer voltage is below ground potential, the first switch is open, so as to allow whatever voltage is provided across the capacitor to be coupled to the non-inverting input of the comparator.
In a similar manner, the inverting input of the comparator is further coupled to a second controlled switch that selectively connects the inverting input of the comparator to ground or remains open in accordance with the polarity of the ringing signal. As with the first controlled switch, when the ringer voltage is at or above ground potential, the second controlled switch is closed, so as to coupled the non-inverting input of the comparator to zero or logic low potential. On the other hand, when the ringer voltage is below ground potential, the second switch is open, so as to allow the measured scaled tip-ring voltage to be coupled to the inverting input of the comparator.
The periodic waveform modulator operates as follows. During the time that the ringer voltage is in its relatively positive state (above ground potential), both inputs to the comparator are grounded (logical xe2x80x98lowxe2x80x99) through the first and second closed switches, so that the comparator output is at a prescribed (built-in) xe2x80x98highxe2x80x99 logic state. When the ringer voltage transitions from its relatively positive state to its relatively negative state, both (closed) switches are opened, and the potential at the inverting input of the comparator rapidly acquires the value of the differential tip-ring voltage. However, the non-inverting input of the comparator initially remains at its zero volt potential, due to the inherent delay associated with the properties of the capacitor. This forces the comparator""s output to transition from its previous logic xe2x80x98highxe2x80x99 state to a logical xe2x80x98lowxe2x80x99 state.
The potential at the comparator""s non-inverting input becomes increasingly positive at a charging rate established by the inherent physics of the capacitor. Within an error introduced by the prescribed (small) built-in offset voltage of the comparator, once the capacitor voltage becomes approximately equal to the measured tip-ring differential voltage, the comparator output transitions from its logical xe2x80x98lowxe2x80x99 state back to a logical xe2x80x98highxe2x80x99 state. Thus, the comparator output produces a periodic waveform having a frequency equal to that of the ringer signal, with its waveform being at a logic xe2x80x98lowxe2x80x99 state for the duration defined by the magnitude of the measured tip-ring voltage. Namely, the duty cycle of the successive pulses of the periodic waveform derived from the ringer signal will be proportional to the tip-ring voltage of the telephone line.